Cathodic protection and underground metallic structure embodying the same



R. H. BROWN CATHODIC PROTECTION AND UNDERGROUND METALLIC STRUCTURE EMBODYING THE SAME Flled Aug 28 1946 INVENTOR. R. Brawn I I '3 MI HTIoRA/ Patented Aug. 2 8, 1951 oA'rnonio PROTECTION A UNDER- GROUND METALLIC STRUCTURE EM- BonYING THE SAME Robert Harry Brown, New Kensington, Pa, as-

signor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application August 28, 1946, Serial No. 693,593

3 Claims.

This invention relates to the cathodic protection of metallic underground structures where the structure to be protected against corrosive action is made the negative electrode (cathode) of a cell, or a multiplicity of cells, in which the anode is formed of a separately buried metal piece or electrode formed of a metal electropositive (anodic) to the metal of which the underground structure is formed. In such installations an electrical connection such as a wire or bus connects the buried electrode to the buried metallic structure, the earth in which the electrode and the structure are buried serving to complete the circuit.

The invention is directed to such cathodic protection installations when the anode of the cell, or cells, is formed of aluminum or an alloy composed principally of aluminum, i. e. containing more than 50 percent by weight of that element, a generic term for such metal and its alloys being aluminous metal. Thus the present invention is concerned solely with the protection of metallic underground structures composed, or composed in important part, of metal or alloy which is electronegative to aluminum. Since most of the important metallic underground structures, including the buried linear installations, such as pipe lines for transporting liquids or gases, and the buried unit installations, such as oil well casings, tower or structural footings and the like, are commonly made of iron, steel or other metal or alloy electronegative (oath,- odic) to aluminum, the aluminous metal anode is adapted to the cathodic protection of most metal.- lic underground structures of commercial importance. While later reference will be made to pip lines since such lines are, at once, a convenient and important commercial form of underground structure, the principles of this invention are .useful whatever the type of metallic underground structure which needs protection against corrosion; provided, of course, that the structure or important parts thereof is made of metal electronegative (cathodic) to an aluminous'anode.

One leading object of this invention is the provision of means and methods by which metallic underground structures may be efficiently protected againstcorrosion by the use of aluminous metal electrodes in the protection circuit. Another leading object of the invention is the provision of corrosion-resistance metallic underground installations comprising a buried structure composed of metal electronegative (cathodic) to aluminum, a separately buried anode composed 2 principally of aluminum, an electrical connection therebetween, and a specially prepared composition surrounding the anode.

As is well known, it is important to the cathodic protection of metallic underground structures that a steady current of relatively constant potential be supplied in the cathodic protection and that this result be achieved at a slow rate of consumption of the metal of the anode. Heretofore the use of aluminum, or alloys composed principally of that metal, in cathodic protection circuits of the type described has met with indifferent success because the circuit did not maintain the desired steady current of constant potential and the anode was consumed at a high rate, or at such a low rate as to provide inadequate cathodic protection. The advantages secured by my invention flow from the provision of a specially prepared composition surrounding the aluminous anode, the combination insuring a steady protective current of fairly constant potential obtained at a lower comparative rate of consumption of the metal of the anode, thus resulting in increased operational efficiency both from the standpoint of the protection obtained and the amount of expended anode metal necessary to such protection.

For convenience in further description of the invention reference will be had to the attached drawing which is a somewhat diagrammatic elevational view, partly in section, of a conventional installation designed to offer cathodic protection to a metallic underground structure. Referring to the drawing, a pipe line I!) is buried in the earth II at a distance below the earth surface 12. At intervals along the run of line H], but at a distance therefrom, electrodes, such as anode is, are buried and connected to the pipe line It by an electric wire or bus, such as cable M. A protective coating I5, which may be asphalt or similar substance, may be used to prevent corrosion at the point between anode l3 and cable 14 and a similar protective coating may be used to protect the connection between cable l4 and line H). To install the anode l3 a hole, such as indicated by the broken line 16, is dug in the earth, the anode is positioned suitably in the hole and then held in place by means of a backfill or mixture I! which is tamped around the anode. The mixture I! usually has an earthen base and is formed by making a water bound sludge of earth and selected chemicals. After the anode l 3 is thus positioned an earthen cap [8 is placed in the hole to surface level. The mixture i! preferably entirely surrounds the anode i3 3 but may, if desired, only surround it in part or contact only one or more surfaces thereof. In each case results are obtained but anode efficiency is greatest where the mixture substantially surrounds the anode.

In the practice of the resent invention the general procedure just described is followed. As above noted, the anode I3 is made of aluminous metal while the pipe line is of some metal, such as iron, which is electronegative (cathodic) to aluminum. The improvement of the present invention consists in providing around the anode a mixture having a pH greater than about 7.5 and containing at least 1 percent by weight of the dry mixture of inorganic halide. The aluminous anode, used in combination with such a mixture, and in such a circuit as that described, functions to maintain a high current output over long periods of time and to effectively protect the metallic underground structure with which it is coupled against corrosive agents. Moreover, the aluminous anode, when used in combination with such mixture, performs this stated function at high efficiency as measured by the amount of aluminous electrode metal consumed per unit of current provided, thereby insuring that necessary replacement of the aluminous anode will be kept at a minimum thereby decreasing upkeep costs.

The base of the mixture may be earth of the type in which the structure is embedded or of any other type, or the base or carrier portion of the mixture may be some other inert material, such as sand, cellulosic material, or an material which will serve as a porous medium or base with which the active chemicals may be mixed and in which they will be retained subject to the leaching effect of ground water or other earth moisture The inorganic halide may occur in the mixture in any form or compound, the compounds of lesser solubility having the advantage of longer withstanding leaching action. To be effective the inorganic halide should be present in amount of at least 1 percent of the dry weight of the mixture. Very large amounts may, if desired, be employed but, since usually maximum effect is reached between about 1 and 30 percent by weight additional amounts are mere surplusage. This is, however, no critical limit and an effective amount of inorganic halide may be said to be an amount of 1 percent or more by weight of the dry mixture. Usually amounts of 5 to percent by weight are to be preferred. Of the halides the chlorides appear to form the most efficient mixtures from the standpoint of maintaining a steady and uniform output of current in the circuit. Usually halide salts of the alkali metals or alkaline earth metals are preferred because of the availability of such compounds and their inability to counteract any compound added to the mixture to maintain the desired alkalinity.

The required pH of the mixture may be obtained by adding thereto an alkaline substance. While the pH should be at least 7.5 I prefer to form mixtures having pH values of not less than 8 or 9 and mixtures of higher pH values are, in general, even more effective. While not understanding fully the mechanism of the mixture in coacting with the aluminous metal anode to maintain a steady uniform output of current in the protective circuit, I am of the belief that to some extent the efiiciencies obtained b the practice of this invention are affected by the relative Solubility of the corrosion product formed at the aluminous anode and therefore I prefer to use a1- kaline substances, such as alkali metal and alkaline earth metal compounds, i. e. oxides, hydroxides, carbonates and the like, which do not appear to have any insolubilizing action in the corrosion product. Such theoretical analysis is at best uncertain and while it seems desirable to adjust the pH of the mixture by the use of alkaline earth metal or alkali metal compounds, any alkaline substance may be used for this purpose with satisfactory results.

Among the aluminous metal anodes which may eb selected for use in the practice of this invention some are more efficient than others. Those containing any substantial amount of copper are least preferred, while those containing substantial amounts of zinc, i. e. about 1 to 20 percent by Weight seem to give the most satisfactory results. Anodes made of pure aluminum or commercial aluminum are satisfactory and where it is desirable to raise the electrode potential of that metal with respect to the metal of the structure which is to be protected best results are obtained by alloying with aluminum at least one of the following class of metals; silver, cadmium, zinc, platinum, calcium, barium, strontium, gallium, indium, bismuth and tin. Any of these metals when present in aluminum in amounts as small as 0.02 percent by weight have some efiect in raising the electrode potential of that metal, but usually amounts of 0.1 percent and more are to be preierred.

In the practice of this invention the mixture which is prepared to surround the anode may, as above indicated, be tamped around the anode after the anode has been positioned in an earth hole of suitable location and size. Alternatively the mixture may be preformed around the anode and the preformed package placed in the earth hole. Preformed anode packages are, however, the invention of others, one type thereof being described and claimed in the application of R. B. Mears, Serial No. 705,508, filed October 25, 1946, and form no part of this invention, except as there may be utilized therein mixtures of the type herein described in combination with aluminous metal anodes.

Examples of the excellent results obtained by the practice of this invention were observed when aluminous metal anodes of varying composition, some being of commercial aluminum and others being of commercial aluminum alloyed with varying amounts of zinc (1 to 10 percent by weight), were installed in soils of various types in Texas in circuit with a steel pipe line. The aluminous metal anodes were, in accordance with this invention, surrounded in installed buried position with mixtures of pH greater than 8 in which alkalinity had been established by the presence of varying amounts of lime ('7 to 15 percent by weight). Each mixture so formed likewise contained 7 to 15 percent by weight of sodium chloride or, in some cases, magnesium chloride. Over the observed six months period these installed electrodes steadily maintained in each circuit, formed by connecting an electrode with an underground pipe line, a flow of current of high potential at calculated efficiencies (ampere-hour output per unit weight) varying from about to about percent.

Having thus described my invention, I claim:

1. In a corrosion-resistant metallic underround installation, in combination, a metallic underground structure composed of metal cathodic to aluminum, at least one underground anode composed of aluminous metal and separate from said structure, an electrical connection between said structure and said anode and a backfill in contact with said anode, said backfill comprising a carrier portion, an effective amount of at least 1 to 30 percent by weight of inorganic halide and sufiicient alkaline material to impart to the backfill a pH in excess of 7.5. v

2. In the method of cathodically protecting an underground metallic structure composed of metal cathodic to aluminum and including the steps of installing an underground aluminous anode and electrically connecting said anode to said structure to form a galvanic cell, the improvement consisting of contacting the aluminous anode with a backfill comprising an effective amount of at least 1 percent by Weight of inorganic halide and having an adjusted alkalinity greater than that indicated by a pH of 7.5.

3. In a corrosion-resistant metallic underground installation, in combination, a metallic underground structure composed of metal cathodic to aluminum, at least one underground anode composed of aluminous metal and separate from said structure, an electrical connection between said structure and said anode and a backfill in contact with said anode, said backfill comprising a carrier portion, an effective amount of at least 1 percent by weight of a halide of an alkaline earth metal or an alkali metal and suflicient alkaline material to impart to the backfill a. pH in excess of 7.5.

ROBERT HARRY BROWN.

6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,244,118 Mullen Oct. 23, 1917 1,856,506 Preston May 3, 1932 1,997,165 Brown Apr. 9, 1935 2,204,823 Rhodes June 18, 1940 FOREIGN PATENTS Number Country Date 2,927 Great Britain of 1884 392,208 Great Britain May 15, 1933 OTHER REFERENCES Corrosion, vol. 1, (1945), pages 59-61, 113- 118.

Chemical Abstracts, vol. 33 (1939), page 2453.

Transactions of The Electrochemical Society, vol. 90 (1946), pages 485, 486, 494, and 499 through 503.

Comprehensive Treatise on Inorganic and Theoretical Chemistry, by Mellor, vol. 5 (1924), page 220.

Status of Cathodic Protection Lines in 1941, by Logan (National Bureau of Standards) page 6.

Certificate of Correction Patent No. 2,565,544 August 28, 1951 ROBERT HARRY BROWN It is hereby certified that error appears in the printed specification of the above numbered patent requ1ring correction as follows:

Column 1, line 52, for corrosion-resistance read corrosion-resistant; column 2, line 7 after protection insert system;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 18th day of December, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

1. IN A CORROSION-RESISTANT METALLIC UNDERGROUND INSTALLATION, IN COMBINATION, A METALLIC UNDERGROUND STRUCTURE COMPOSED OF METAL CATHODIC TO ALUMINUM, AT LEAST ONE UNDERGROUND ANODE COMPOSED OF ALUMINOUS METAL AND SEPARATE FROM SAID STRUCTURE, AN ELECTRICAL CONNECTION BETWEEN SAID STRUCTURE AND SAID ANODE AND A BACKFILL CONTACT WITH SAID ANODE, SAID BACKFILL COMPRISING A CARRIER PORTION, AN EFFECTIVE AMOUNT OF AT LEAST 1 TO 30 PERCENT BY WEIGHT OF INORGANIC HALIDE AND SUFFICIENT ALKALINE MATERIAL TO IMPART TO THE BACKFILL A PH IN EXCESS OF 7.5. 